Technical Field
The present invention relates to a method for preparing methacrolein by the reaction of t-butanol or a t-butanol-containing gas with molecular oxygen or a molecular oxygen-containing gas in a fixed bed reactor filled with catalysts.
Background
The process for the preparation of methacrolein from t-butanol or a t-butanol-containing gas comprises two chemical reactions, one is t-butanol being dehydrated to form isobutene, and the other is isobutene being oxidized to form methacrolein. Since such an oxidation reaction is a strong exothermic reaction, there is an area, i.e., hot spot, within the catalyst bed where the heat is intensively released. The temperature at the hot spot is very high, leading to an excessive oxidation reaction, accelerating thermal cracking of the catalyst, or even leading to temperature runaway phenomenon. Several solutions have been reported to solve the above problems. For example, it has disclosed a method for producing methacrolein and/or methacrylic acid by using t-butanol as a starting material, comprising supplying t-butanol to a fixed-bed multi-tubular reactor having a dehydration catalyst layer and an oxidation catalyst layer for dehydrating t-butanol to produce isobutene, and oxidizing isobutene to produce methacrolein and/or methacrylic acid. The dehydration catalyst layer comprises a catalyst containing alumina or alumina-silica and the filling length of this catalyst is from 3 to 20% of the filling length of the oxidation catalyst layer. This filling manner results in shortages, such as, extending the total length of the filler within the bed and, in turn, increasing the pressure drop during reaction, which may lead to increased temperature in the hot spot within the oxidation catalyst bed and decreased reaction selectivity.
The prior art has disclosed a method for producing methacrolein by using isobutene as a starting material, comprising a two-step catalytic oxidation during reaction by using two reactors in series. The first reactor is filled with an assistant catalyst having low activity and high selectivity and the second reactor is filled with a main catalyst having high activity and high selectivity. However, this filling manner increases the numbers and sizes of the reactors, resulting in increased investment of device and decreased economical efficiency of the project.
The prior art has further disclosed to fill the catalysts having different catalytic activities in upper zone and lower zone of a reaction tube such that the catalytic activities thereof can be gradually increased along the direction from the gas inlet to the gas outlet. However, since the temperature of the raw gas introduced into the catalyst layer in a gas-solid phase catalytic oxidation is generally lower than the reaction temperature, the raw gas needs to be pre-heated to obtain an effective catalytic efficiency. It is conventional to provide a pre-heating zone at the inlet of the raw gas. However, if the pre-heating zone is not provided or such a zone is relatively short, the raw gas, which has relative low temperature, rarely proceeds with the reaction when contacting with the catalyst having lower activity in the inlet. This is not economic because of the expensive catalyst being function as a medium of pre-heating zone. In addition, even if there is a pre-heating zone, the reaction temperature at the inlet of the bed will still be relative low if t-butanol is used as a starting material, as dehydration of t-butanol to isobutene is an endothermic reaction. Under this low temperature, the reaction efficiency will be very low if the catalyst having low activity is used to contact the starting material.
Therefore, there is still a need to develop a method to produce methacrolein by using t-butanol as a starting material, which method can effectively inhibit the formation of hot spot in the catalyst layer during the oxidation of methacrolein and, at the same time, can increase the utilization efficiency of the catalyst layer around the gas inlet.